The present invention relates to a low IF receiver, which expression is intended to cover a low IF receiver section of a transceiver. Such a receiver is suitable for fabrication as an integrated circuit for use in telecommunications products, such as cellular and cordless telephones, and TV tuners.
Low IF receivers are frequently implemented as polyphase or sequence asymmetric receivers. FIG. 1 of the accompanying drawings illustrates a typical polyphase receiver. A signal received by an antenna 10 is applied to first inputs 12, 13 of quadrature related mixers 14, 15. A local oscillator 16. generating a local oscillator frequency which is offset typically by half a channel spacing from the nominal carrier frequency of the received signal is applied by way of a ninety degree phase shifter 17 to a second input of the mixer 14 and directly to a second input 19 of the mixer 15. The outputs of the mixers 14, 15 are the I and Q channels, respectively. These channels are applied to a polyphase filter 20 functioning as a band pass filter. The outputs of the polyphase filter 20 are applied to respective inputs of a demodulator 22 which provides an output, audio frequency signal on an output terminal 24.
From FIG. 1 it can be seen that the basic polyphase receiver structure is very similar to a zero IF receiver in that the IF consists of a quadrature pair of channels, the in-phase or I channel and the quadrature-phase or Q channel. However, unlike the zero-IF receiver where the local oscillator is run at the centre frequency of the wanted signal, in the polyphase case the low-IF is achieved by running the receiver local oscillator at typically half a channel spacing from the centre frequency of the wanted signal. Thus for example in GSM (Global System for Mobile Communications) with a channel spacing of 200 kHz, if the desired signal is at 950 MHz then the local oscillator could be at 950.1 MHz giving an IF of 100 kHz. The principal advantage of the polyphase approach over the zero-IF approach is that large DC offsets which are unavoidably generated by the two mixers are no longer in the IF bandwidth and therefore do not interfere with the demodulation of the wanted signal.
From the example frequencies given it is obvious that any signal present at 950.2 MHz will form an image signal also at an IF frequency of 100 kHz. In principle this is not a problem since the polyphase filter is able to distinguish signals on its complex inputs having opposing phase relationships, as is the case between the desired and the image frequency. However, this image rejection capability is in practice limited by the level of matching and balance that can be achieved in amplitude and phase between the I and Q channels, which means that an extremely strong signal at the image frequency will cause interference to the desired signal in the IF if the desired signal is too weak. Such a problem does not appear too serious for the DECT and GSM systems because there is a relatively relaxed specification on the adjacent channel rejection required by a receiver. However if the polyphase approach is extended to other systems not having such a relaxed specification on the adjacent channel rejection then image rejection will become more of an issue.
An object of the present invention is to improve image rejection in low IF receivers.
According to one aspect of the present invention there is provided a low IF receiver comprising frequency down conversion means including a local oscillator, a channel selectivity polyphase filtering means for selecting a wanted channel signal from the output of the frequency down conversion means, means for detecting when the quality of the frequency down converted signal becomes unacceptable and means responsive to said detection for changing the local oscillator frequency and the centre frequency of the polyphase filter.
More particularly the present invention provides a low IF receiver comprising input signal receiving means, quadrature related frequency down conversion means including first and second mixing means having first inputs coupled to the input signal receiving means and second inputs coupled to a local oscillator means for generating a local oscillator signal having a frequency offset from a nominal centre frequency of the input signal, a channel selectivity polyphase filtering means for selecting a wanted channel signal from outputs of the first and second mixing means, means for detecting when the quality of the frequency down converted signal becomes unacceptable and means responsive to said detection for changing the local oscillator frequency and the centre frequency of the polyphase filter.
In one embodiment of the present invention the means for changing the local oscillator frequency also inverts the sign of one of the local oscillator signals. The inversion of one of the local oscillator signals effectively tunes the polyphase filter by twice the IF.
In another embodiment of the present invention the detecting means monitors the potential image frequencies and causes the local oscillator or frequency changing means to change the local oscillator frequency in response to deciding which image frequency to use.
The detecting means may comprise a wideband polyphase filter, means for identifying the frequencies present in the output of the wideband polyphase filter, and image frequency analysing means for providing a control signal for altering the frequency of the local oscillator and the centre frequency of the channel selectivity polyphase filtering means and the wideband polyphase filter.
By being able to simultaneously alter the local oscillator frequency and the centre frequency of channel selectivity filter and other complex signal processing means, such as signal de-rotation rate, it is possible to allow the image frequency to be dynamically steered away from that of any unwanted strong signals. This feature can be used to enhance the performance of inexpensive receivers so that they can meet more stringent type approval requirements or alternatively enable high performance products to be built which would have an on-the-air performance better than that which can be met with conventional superheterodyne and homodyne designs.
According to another aspect of the present invention there is provided an integrated circuit comprising quadrature related frequency down conversion means including first and second mixing means having first inputs for an input signal and second inputs for a local oscillator signal having a frequency offset from a nominal centre frequency of the input signal, a channel selectivity polyphase filtering means for selecting a wanted channel signal from outputs of the first and second mixing means, means for detecting when the quality of the frequency down converted signal becomes unacceptable and means responsive to said detection for providing a control signal for changing the local oscillator frequency and the centre frequency of the polyphase filter.
According to a further aspect of the present invention there is provided a method of operating a low IF receiver, comprising the steps of monitoring the quality of a frequency down-converted signal; if the signal quality is unacceptable, changing the local oscillator frequency by the equivalent of a channel bandwidth; determining if the signal quality is better; and if not, reverting back to the prior local oscillator frequency.